Method for deforming metal single crystals



United States Patent 3,413,204 METHOD FOR DEFORMING METAL SINGLE CRYSTALS Ernst Rexer and Karl Schlaubitz, Dresden-bad, Weisser Hirsch, and Erich Zedler and Dieter Miiller, Dresden, Germany, assignors to Deutsche Akademie der Wissenschaften zu Berlin, Berlin-Adlershof, Germany No Drawing. Filed Dec. 21, 1964, Ser. No. 445,293 6 Claims. (Cl. 204-140.5)

ABSTRACT OF THE DISCLOSURE A method for deforming single crystals of body centered cubic metals by mechanical treatment, particularly by rolling, in appropriate crystallographic orientation, i.e. a favorable combination of the rolling direction and the plane of rolling, in small individual steps, whereby a deformed single crystal is obtained which will not recrystallize upon heating to a temperature near the melting point and will not tend to embrittlement.

The present invention relates to deformed metal single crystals, more particularly such crystals which do not recrystallize at a temperature near the melting point and do not exhibit brittleness at the grain bounderies.

By deforming in this context we understand deforming by a mechanical treatment, such as rolling, drawing, flow spinning, etc.

It is known that some metals, e.g. tungsten and mo lybdenum, after having undergone deformation, become brittle at the large angle grain bounderies when annealed to about the temperature of recrystallization. The brittleness of the metals is due to the high temperature treatment and it detracts considerably from their technical use.

Attempts have therefore been made to raise to a higher degree, the temperature at which recrystallization takes place, by certain additives. In the tungsten technology, more particularly, it was possible to raise the temperature of recrystallization by several hundred degrees by the addition of aluminum, silicon, potassium, and thorium. But even such doped tungsten wires become brittle at 2000 C. in spite of the fact that the melting point of tungsten lies at 3400" C.

Brittleness can be counted-acted in tungsten and molybdenum by addition of rhenium, but this is a very uneconomical method.

Other attempts to improve the ductility of metals tending to embrittlement comprise increasing the degree of their chemical purity, the use of agents causing grain refining or desoxidation, or processes for improvement grain size, or other processing measures. Although certain results were obtainable by these attempts, the fact of recrystallization with simultaneous embrittlement at the grain bounderies appear to be unavoidable.

It is the object of the present invention to provide a method for making deformed metal single crystals, which do not recrystallize when heated to the vicinity of the melting point, and which do not exhibit the undesirable phenomenon of embrittlement at the grain bounderies.

Other objects and advantages of the present invention will become apparent from the following detailed description.

The present invention is based on the discovery that the phenomenon of embrittlement can only be completely avoided when the cause therefor is eliminated, the cause being large angle grain bounderies. For this purpose, it is necessary to start from a metal which does not have any large angle grain bounderies, or, in other words, is in the form of a single crystal. Furthermore, care has to be taken that, in a heat treatment which follows deformation, no large angle grain bounderies are created which means that primary recrystallization will not take place.

According to the invention, the condition is fulfilled when single crystals which have suitable crysallographic orientations, are subjected to deformation by mechanical treatment, such as rolling, drawing, flow spinning, and the like, in small steps.

By suitable crystallographic orientations we understand where rolling is concerned, favorable combinations of the crystal axis (rolling direction) and the plane of rolling. In body-centered cubic metals, for instance the combination of the crystal axis 110 and the rolling plane is best, but deformation can also take place in a number of other orientations.

In other deformation processes than rolling, e.g. in drawing, it may be sufficient to select a favorable crystal axis alone.

Before deformation is carried out by rolling, the rolling planes may be ground and/or polished.

The behavior during deformation differs widely from metal to metal and depends, for a given metal, to a considerable degree on the purity of the metal. With in creasing purity, every metal becomes more ductile, that is, more easily deformable. As regards the impurities, there again the ductility of various metals is impaired in a different manner by given impurities. The decrease in ductility may sometimes occur even when the content of impurity is so small that an analytical determination of the same and thereby a dependable indication of the degree of purity of the metal is quite difficult.

With the best methods presently available, e.g. fusion by means of electron bombardment, some metals, for instance, niobium or tantalum, can attain such an improve ment in ductility, that is to say deformability, that they may be easily deformed at room temperature (about 20 C.). Other metals such as molybdenum and tungsten, exhibit a far less favorable deformability, despite high purity, whereby the solubility of the impurities in the crystal lattice, the formation and distribution of precipitations, etc., play an important role. With these metals, a deformation of the polycrystalline metals can only be carried out above room temperature, since every metallic element becomes softer and thereby easier workable at higher temperature.

Which temperature is chosen for deformation in an individual case depends on several factors: On the metal, on its purity, but also on the character of the deformation method and distortions thereby produced in the crystal lattice. For reducing lattice distortions, not only the use of higher operating temperatures, but also that of small steps of deformation is indicated. Finally, it will be a question of available equipment which one of the determining factors is used, that is to say whether the deformation process is favorably affected by higher temperature, or by smaller deformation steps, or by both.

In order to decrease any work-hardening which sets in during the deformation process and favors recrystallization of the finished product, intermediate annealing may take place at elevated temperatures and suitable surrounding atmosphere, preferably in vacno or in a protective gas. What may be the appropriate temperature, depends on the factors outlined above. As to the protective gas, any atmosphere will be considered suitable which does not deteriorate the properties of the metal and would make it less suitable for later use. More particularly, all noble gases have proved suitable, but some other gases or gas mixtures may :also be used. I

When the mechanical deformation process leads to a considerable destruction of the crystal structure in the surface layer of the single crystal, this may cause the formation of nucleus in the later heating treatment and initiate recrystallization. In such cases, the surface layer has to be removed after deformation. The thickness of the layer to be removed depends on the foregoing deformation of the metal. Accurate control is possible by X-ray, in proceeding with the removal until the X-ray picture shows clear Laue reflections.

Due to the fact that the recrystallization is prevented or counter-acted according to the invention, it is possible to make ductile welding joints preferably by welding with an electron beam.

It is an advantage of the deformed metal single crystals prepared according to the invention that they do not become brittle at temperatures near their melting point and that they can be used accordingly in many fields of industrial application, for instance, in electronics, vacuum technique, and similar fields.

In the following, the invention is more fully illustrated by way of two examples, but it should be understood that the process according to the invention is not limited thereby.

EXAMPLE 1 A molybdenum single crystal was made by zone melting by means of electron bombardment in accordance with the process described by M. Davis, A. Calverley, and R. F. Lever, J. Appl. Phys., 27 (1956), pages 195-196. In this process, the metal rod arranged as anode is axially passed through a ring-shaped cathode. The electrons which are accelerated by the cathode toward the rod are capable of melting a narrow zone thereon, which migrates from one end of the rod to the other due to the movement of the same. By passing the metal rod once or several times through the zone melting step, the metal rod is converted into a single crystal.

A molybdenum single crystal thus made having a seeded axis orientation of 110 was ground in the rolling plane {100} and polished and rolled at room temperature in the direction of the axis.

The rolling at room temperature is performed in stages of 0.003 mm. per step. Subsequently to the rolling process, the surface layer was electrolytically polished off. The following annealing in vacuo close to the temperature of the melting point did not result in recrystallization. The metal retained its complete ductility.

EXAMPLE 2 A tungsten single crystal made by electron bombardment zone melting (see Example 1) with na axis orientation of 110 was ground and polished in the rolling plane 100 and rolled at 400 C. in air in the direction of the axis.

The rolling at 400 C. is performed in stages of 0.01 mm. per step. After having raised thereby the hardness to about 420 VPN, intermediate anneals were carried out in vacuum for one hour at 1600 C. Thus the hardness was lowered to its original value.

Subsequently to a thickness reduction of 60% by rolling, the surface layer was electrolytically polished off. The following annealing in vacuo to 3000 C. did not result in recrystallization.

What we claim is:

1. A method for deforming a singlie crystal of molybdenum whereby the deformed crystal obtained will not recrystalize upon heating to a temperature near the melting point and, accordingly, will not become embrittled upon said heating, comprising rolling in the direction of the axis of a single crystal of molybdenum having an axis orientation of l10 and a rolling plane orientation of at room temperature and in stages each of 0.003 mm., and then removing the face of the rolled surface.

2. A method according to claim 1, in which said face is removed by electrolytic polishing.

3. A method for deforming a single crystal of tungsten whereby the deformed crystal obtained will not recrystal lize upon heating to a temperature near the melting point and, accordingly, will not become embrittled upon said heating, comprising rolling in the direction of the axis a single crystal of tungsten having an axis orientation of and a rolling plane orientation of {100}, at a temperature of 400 C. and in stages each of 0.01 mm., and then removing the face of the rolled surface.

4. A method according to claim 3, in which said face is removed by electrolytic polishing.

5. A method according to claim 3, in which said stages of rolling are repeated until the thickness of the crystal is reduced by 60%.

6. A method according to claim 3, further comprising annealing the crystal, to lower its hardness to the original value, after said rolling has been completed and before said face is removed.

References Cited UNITED STATES PATENTS 2/1927 Koref et a1. 148-115 X FOREIGN PATENTS 737,950 10/1955 Great Britain.

OTHER REFERENCES CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,413 ,204 November 26, 1966 I Ernst Rexer et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 10, "axis of" should read axis Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Office;- Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

